Allow tempest cleanup delete resources based on prefix

A warning in command description of run cleanup first with dry-run
is added. The cleanup behavior is extended to allow users to delete
only resources if their name starts with a certain prefix.

Closes-Bug: #1945082
Change-Id: I65dfe051c891b3679538acec713e8616746c47f6
diff --git a/roles/tempest-cleanup/README.rst b/roles/tempest-cleanup/README.rst
index d1fad90..d43319c 100644
--- a/roles/tempest-cleanup/README.rst
+++ b/roles/tempest-cleanup/README.rst
@@ -40,6 +40,12 @@
    some must have been leaked. This can be also used to verify that tempest
    cleanup was successful.
 
+.. zuul:rolevar:: run_tempest_cleanup_prefix
+   :default: false
+
+   When true, tempest cleanup will be called with '--prefix tempest' to delete
+   only resources with names that match the prefix. This option can be used
+   together with dry_run.
 
 Role usage
 ----------